The Cyclic Expression Of Calcium-activated Potassium Channels In Human Endometrium

Background Embryo implantation is a complex process. In the case of implantation, a highly coordinated process is set into motion whereby specialized cells of the embryo, the trophectoderm and trophoblast, establish contact with a specialized tissue of the mother, the uterus. In most mammals ,there is only a restricted time, named "window of implantation", during the uterine cycle during which implantation can occur. In human, the WOI is from day 20 to day 24 during the normal menstrual cycle. Failure to initiate the critical early events of implantation during this WOI results in early pregnancy failure. The mechanism of embryo implantation is still unclear by now. But many molecules, ions, and ion channels are involved in this process.Calcium-activated potassium channel (K_Ca) is a group of K⁺ channel family. On the basis of single channel conductance, K_Ca can divided into large-conductance Ca²⁺-activated K⁺ channel (BK_Ca), intermediate-conductance Ca²⁺-activated K⁺ channel (IK_Ca) and small-conductance Ca²⁺-activated K⁺ channel (SK_Ca).The native BK_Ca channel is composed of four α - and four β -subunits, and the single channel conductance is >100pS. BK_Ca channel can be activated by membrane potential and internal Ca²⁺. The a subunit, encoded by the Slo gene, is thepore-forming portion of the channel and consists of 11 hydrophobic domains(S0-S10). Similar to other voltage-gated channel (such as Kv), the S4 domain contains several positively charged amino acids and most likely represents the voltage sensor of the channel. BK_Ca channel has been extensively studied in the brain, cochlea, and muscle, and alternate splicing of its mRNA is known to produce considerable functional diversity. These splice variants might explain various BK_Ca channel properties reported in different tissues. This variability is expanded through the association of BK_Ca with different β -subunits.Another class of K_Ca channel with a smaller unitary conductance (10-40pS) was first identified in brain, referred to as SK_Ca channel. Up to four members of that highly conserved family are now identified (SK1-SK4, encoded by KCNN1-KCNN4 genes) including a K_Ca channel that was previously identified in erythrocytes and lymphocytes as an IK_Ca channel based on single channel conductance and a distinct pharmacology. However, IK_Ca is now referred to as SK4 due to a high degree of similarity with the other SK_Ca channel. A functional SK_Ca channel results from the coassembly of four subunits, each of them consisting of six transmembrane domains (S1-S6) with intracellular NH₂ and COOH terminals. It has been suggested that different subunits (SK1-SK3) could interact to form heteromultimers. Interestingly, this channel family lacks voltage sensitivity even with the presence of several charged amino acids in the fourth membrane-spanning domain. The expression pattern of SK_Ca channels is quite different from the BK_Ca channel. In fact, SK_Ca channels are expressed in neurons, endothelium, epithelium, and several types of smooth muscles. Opening of the SK pore is independent of the membrane potential but strictly depends on [Ca²⁺]_i. The calcium sensitivity of SK channels depends on CaM.K_Ca channels play important roles in many processes involving Ca²⁺-dependentsignaling in both electrically excitable and nonexcitable cells. The distribution of K_Ca channels is ubiquitous, with variable functions such as regulating smooth muscle's excitability, vascular tone, cell's proliferation, differentiation, apoptosis and secretion. In female reproductive system, BK_Ca channels have only been found expressed in myometrium in previous studies, associating with the initiation of uterine contractility during parturition. There is no information on K_Ca expression in human endometrium.Objective The aim of this study is to investigate the expression levels and characteristics of K_Ca in human endometrium during the normal menstrual cycle.Material & methods Immunohistochemistry assay was used to detect the location of K_Ca in human endometrium. RT-PCR was utilized to detect the mRNA expression levels of K_Cain endometrial samples semi-qualitatively.Results BK_Ca, SK1, SK2 and SK3 mRNA were detected in all human proliferative and mid-secretory endometrium. Semi-quantitative analysis of mRNA showed that the expression level of BK_Cain the proliferative phase was significantly lower than that in the mid-secretory phase, the expression level of SK1 in the proliferative phase was significantly higher than that in the mid-secretory phase, whereas there was no significantly difference of SK2 and SK3 mRNA expression. IK_Ca mRNA were detected only in 2 of 30 samples. BK_Ca and SK3 protein was located in the glandular epithelial cells and the luminal epithelial cells.Conclusions We reported in the first time that K_Ca is expressed in human endometrium. The phase related mRNA expression of BK_Ca and SK1 in endometrium during menstrual cycle might indicate its potential function on endometrial receptivity.